Electronics are playing ever-increasing roles in downhole applications such as testing, measurement, control, actuation, communication, etc. A tool is a functional unit which may include electronics, sensors and other components to execute particular function(s). A bottom hole assembly (BHA) is a group of segments (usually in tubular or cylindrical form) connected end-to-end, and has a collection of tools installed inside the segments. In a BHA, a segment, usually made of a metal or alloy, is a structural unit and is used to protected sensitive electronics or sensor against harsh downhole conditions. A segment may include part of a tool, only one tool, or multiple tools. A tool may be divided among multiple segments which are adjacent or interleaved with other segments. A tool bus physically includes a number of continuous electrical links and is used to interconnect the tools together in order to exchange information and share electrical power internally or externally to the surface. A node is a tool access point which is electrically coupled to the tool bus. A tool may have one node, multiple nodes or share one node with other tools. Electrical links usually made up of wires, components and connectors, etc. pass through the segments and extend across their joints all the away along the BHA in either a direct-contact or a contactless manner to create the physical communication channels for the tool bus.
In the drilling process, BHA tools work together closely to ensure fast penetration, superior borehole quality and precise wellbore placement while maintaining the integrity of the drillstring. A variety of measurements may be continuously taken while drilling, which include natural gamma ray, electrical signals, acoustic signals, stimulated radioactive response, electromagnetic signals, nuclear magnetic resonance, pressure and other properties of the rocks and their contained fluids, etc. Various electronics and sensors are installed in sealed chambers distributed axially in the segments of the BHA, and are further divided by functionality and grouped together as different tools.
These tools are electrically connected to one common tool bus. Normally, one tool is elected as the bus master to manage the bus access, to control or coordinate the BHA operation, and to communicate with the surface through one telemetry tool. As the result, a multipoint bus with a single master and multiple slaves is created. Therefore, information can be exchanged within BHA tools or with the surface under the control of the bus master. Electrical power, usually DC power from a battery or a turbine generator is shared among the tools.
Typically, a BHA communicates with the surface through a mud pulse or electromagnetic (EM) telemetry tool which have a very low speed of a few bits per second, so only a small amount of critical data is communicated real-time while most of downhole data is stored in tool memory. Once the BHA is brought back to the surface, the stored data is traditionally retrieved and processed by powerful surface computers. Until then, in-depth understanding of formations and comprehensive knowledge of trapped fluids cannot be developed.